Global Climatology — Page 377, Lesson 351

I want to walk you through the July pressure patterns and then the surface wind systems that define global climatology. Let's start with July. In July, the Southern Hemisphere pressure pattern stays close to the ideal model. Overland temperatures there are colder, so the subtropical high pressure belt is generally unbroken — it remains a continuous band of high pressure. Now, the equatorial low pressure zone — the belt of low pressure along the thermal equator — lies to the north of the Equator in July. That's because the sun has shifted northward, heating the Northern Hemisphere landmasses more intensely. Here's the big change: where we would expect subtropical highs in the Northern Hemisphere, low pressure areas now form over the land masses due to solar heating. So the Siberian High that dominated in January is replaced by the Baluchistan Low, centred over Pakistan but affecting all of Asia. North America also has low pressure instead of a subtropical high. Meanwhile, the Aleutian Low and the North Australia Low both disappear in July. The Icelandic Low — which is a statistical low pressure area — becomes less deep and disperses into three small areas: off Greenland, the Baltic, and Iceland, all at 1010 hectopascals. The Azores High and the Pacific High become dominant in July, both at 1025 hectopascals. These are the subtropical high pressure cells over the oceans. Let's move to surface winds. First, the Westerlies of Temperate Latitudes. These westerly winds exist in the region between the subtropical highs and the temperate lows, roughly from 40° to 60° latitude. They are caused by the turning effect of geostrophic force — that's the Coriolis force — acting on the poleward outflow from those subtropical highs. In the Northern Hemisphere, the westerlies apply mainly over the oceans, with frequent winter gales. During summer, these westerlies are less constant and less strong. In the Southern Hemisphere, these winds are largely uninterrupted by land masses and are consequently strong. They are called The Roaring Forties because they blow principally between latitudes forty and fifty South. Weather in this belt comes from rapidly moving depressions — wild weather, strong westerly winds and gales, overcast skies and heavy rain. Underneath the subtropical high pressure zones, the wind speed is relatively slow and sometimes nonexistent. The areas between 30° and 40° North have become known as the "Horse Latitudes." That name comes from the time when sailors en route to the Americas disposed of their horses off the ships rather than have to feed them when the sailing ships were becalmed in these light-wind zones. Now, Trade Winds — also called Tropical Easterlies. Trade winds are consistent winds converging into the equatorial trough from the subtropical high belt on each side of it. The turning effect of geostrophic force — again, Coriolis — causes northeast trades in the Northern Hemisphere and southeast trades in the Southern Hemisphere. The trade winds blow towards the thermal equator and will therefore change direction when crossing the geographic equator. Northeast trades will back — that means they shift anticlockwise, turning more westerly. Southeast trades will veer — they shift clockwise, also turning more westerly. Fine weather prevails in the poleward and eastern parts of the tropical oceans, while towards the west and the Equator, unstable conditions dominate, with cloudy, showery weather.